Producing images of high diagnostic quality using low dose by scrutinizing protocol

The following case was submitted by a participant for Radiographer of the year 2017 contest sharing his best practices for radiation dose management.

Describe your focus of study

Head computed tomography protocol audit and remediation to manage dose.

Indicate the purpose and objective of the case

Variations in computed tomography (CT) dose output are often noted in clinical practice, due to differences in local scan protocols. Computed tomography protocols have extensive choice of adjustable dose saving features that have been proven to effectively manage the dose without detriment to the diagnostic quality of the CT images when properly used. Sadly, many Radiographers are content with default protocols installed by the CT manufacturers or programmers, irrespective of the body habitus of the patient. This work targets a foremost reference hospital with considerable patient throughput. The purpose was to review and correct likely weaknesses in default head CT protocols in order to manage dose to the patient.

Provide brief clinical and patient background

One hundred ward and outpatients referred for CT investigations were randomly selected from a pool of 654 patients who were attended to at the center. These patients were adults aged 18 - 93 years with weight range of 60 to 80 kg. Some were bedridden but the majority were ambulant. They were divided into two groups, A and B, based on weight. The weights in group A had counterparts in B.

DoseWise radiographer must intuitively have empathy for their patients. This is achieved by scrutinizing protocol parameters, and both prospective and retrospective dose information, for every patient.

Describe the dose management methods/techniques used

A protocol is efficient if it manages dose while producing images with high diagnostic quality. The efficiency of CT protocols is determined by two dosimetric quantities at the end of each scan, per International Electrotechnical Commission requirements (IEC); CTDIvol for a single section and, DLP for the entire examination.

The first group of patients (A) were examined using standard head technique as well as with the default protocols used by the Radiographers at the center. The CTDIvol and DLP were extracted from the monitor and the 75th percentile were calculated. Subsequently, there was a scrutiny of tube current (mA) and modulation option, tube potential (kVp), gantry rotation time (seconds), scan range for cranial exams (mm), pitch, and azimuth for supine scanogram. The values of the parameters set in the protocol was noted to for ease of reversal if need be. The kVp was reduced from 140 to 120. The mA was moved from 250 to a range of 180 to 220 using automatic mA modulation. Gantry rotation time was moved from 1 sec to 0.7 s. Scan range was adjusted to 140 mm from 250 mm. The pitch of 0.75 was replaced with 1.5 to minimize gantry rotations/ series. Cranial examinations were done supine as recommended but at an improper azimuth of 0 and 90 degrees. This corresponds to anteroposterior (AP) and lateral scanograms. The AP azimuth would increase eye and thyroid dose so they were reprogrammed to 90 and 180 degrees to represent lateral and postero-anterior scanograms, respectively.

The second group of patients were then examined after this adjustments. A second calculation of the 75th percentile of CTDIvol and DLP was done for this group B patients who shared similar weight characteristics as group A. Feedback on image resolution was got from Radiologists who were all blinded to the study. Both pre- and post-intervention values were compared with the 60 mGy (CTDIvol) and 1050 mGy-cm (DLP) recommended by the European Commission.

Explain the results and conclusions reached

The pre-intervention CTDIvol and DLP outputs were 65 mGy/1634 mGy-cm (group A) while the post-intervention values were 58 mGy/986 mGy-cm (group B). These were 3.3% (CTDIvol) and 6. 1% (DLP) lower than the recommendations of the European Commission. The high dose was as a result of reliance on default protocol installed by the supplier of the scanner. Radiographers ought to scrutinize protocols before using them. The feedback from a 'popping' prospective dose information during axial scan planning must not be ignored. For optimization of patient protection during CT procedures, the tube current (mA), tube potential (mA), gantry rotation time (s), pitch and azimuth should be meticulously preset.

The dual goal of medical imaging anywhere is to produce images of high diagnostic quality using doses that are as low as reasonably achievable (ALARA). Digital technology such as CT lead to dose creep due to the pressure to avoid quantum noise and hence, repeats. But a DoseWise Radiographer must intuitively have empathy for their patients. This is achieved by scrutinizing protocol parameters, and both prospective and retrospective dose information, for every patient.

A starting point is to programme the scanner for automatic tube current modulation, a software-based tip for crashing dose. It is based on the premise that pixel noise is attributable to quantum noise in the projections. By adjusting the tube current to follow the changing patient anatomy, quantum noise can be adjusted to maintain the desired noise level. A kVp of 120 is often adequate for head exams.

In conclusion, no Radiographer should attempt to use a CT scanner without first ascertaining the protocol parameters. Absolute reliance on default protocols is not DoseWise as shown by our results. Keen attention to prospective and retrospective dose information are indispensable in radiation protection during CT procedures.

Discuss the case outcome(s), future implications, etc.

It is necessary to have two important feedback during protocol design and or, adjustment. The first is the CTDIvol and DLP output, and the second is an independent assessment of image quality. This second aspect is very necessary in order to avert a situation of wrong programming leading to considerable quantum noise and hence, repeats. This would then neutralize the benefits of optimization. In this instance, the Radiologist gave the feedback on image quality.

The 75th percentile of the CTDIvol and DLP for a sizeable number of patients from the CT scanner should be calculated and where the dose output is higher than international recommendations, protocol correction should be initiated. This is particularly needful now that the general public are becoming increasingly aware of radiation carcinogenesis.

It is recommended that CT manufacturers also collaborate with Radiographers on dose management.

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